In general, regulated emissions for internal combustion (IC) engines include carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulates. However, such regulations have become more stringent over recent years. For example, the regulated emissions of NOx and particulates from diesel-powered IC engines are low enough that, in many cases, the emissions levels cannot be met with improved combustion technologies alone. To that end, exhaust after-treatment systems are increasingly utilized to reduce the levels of harmful exhaust emissions present in exhaust gas.
Conventional exhaust gas after-treatment systems include any of several different components to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust after-treatment systems for diesel-powered IC engines include various components, such as a diesel oxidation catalyst (DOC), a selective catalytic reduction (SCR) catalyst, a diesel particulate filter (DPF), an SCR on filter (SCRF), and/or an ammonia slip catalyst (ASC) (also referred to as an ammonia oxidation catalyst (AMOX)). Each of the DOC, SCR catalyst, DPF, SCRF, and/or the ASC components are configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through or over the respective components.
Generally, DOCs reduce the amount of CO and HC present in the exhaust gas via oxidation techniques, as well as to convert NO to NO2 for passive regeneration of soot on a DPF and to facilitate fast SCR reactions. DPFs filter harmful diesel particulate matter and soot present in the exhaust gas. SCR catalysts and SCRFs have been developed to remove NOx from the exhaust gas, which is relatively more difficult to remove than CO, HC and particulate matter.
SCR catalysts are configured to convert NOx (NO and NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O). A reductant (typically ammonia (NH3) in some form) is added to the exhaust gas upstream of the catalyst. The NOx and ammonia pass over the catalyst and a catalytic reaction takes place in which NOx and ammonia are converted into N2 and H2O. An SCRF is an assembly that performs the combined functions of an SCR and a DPF.
In most conventional SCR and SCRF systems, ammonia is used as a reductant. Typically, pure ammonia is not directly used due to safety concerns, expense, weight, lack of infrastructure, and other factors. Instead, many conventional systems utilize diesel exhaust fluid (DEF), which typically is a urea-water solution. To convert the DEF into ammonia, the DEF is injected into a decomposition tube through which an exhaust stream flows. The injected DEF spray is heated by the exhaust gas stream to trigger the decomposition of urea into ammonia. The exhaust gas mixture including the ammonia decomposed from the urea further mixes while flowing through the decomposition tube and passes over the SCR catalyst (e.g., SCR “brick”), where the NOx and ammonia are converted to N2 and H2O.
For optimal SCR and SCRF systems, a specific amount of reductant is applied to the exhaust gas at each instant such that all of the ammonia decomposed from the reductant, or applied directly to the exhaust gas, converts all of the NOx present in the exhaust gas at that instant to N2, H2O, and a small amount of N2O. If too little ammonia is applied, the exhaust gas will retain high levels of NOx. However, if more ammonia is applied than is necessary to convert all of the NOx, ammonia leakage, or “slip” from the SCR catalyst may occur. In this case, the ammonia slip either converts to N2 and NOx over the ASC or is discharged (e.g., “slips”) from the tailpipe into the environment. Ammonia slip is undesirable because ammonia and NOx are caustic and harmful to the environment. In addition, ammonia slip indicates that excessive reductant is being dosed, which is wasteful and inefficient, resulting in higher operating costs.
Conventional exhaust after-treatment systems utilize various control methods for controlling the operation of a reductant doser in an after-treatment system. However, conventional systems have slow response times and perform poorly in cold temperatures and during transient periods. Thus, there is a need for improved exhaust after-treatment systems.